The invention relates to a process for the preparation of polyvinyl alcohol articles of high tensile strength and high modulus.
Synthetic fibres on the basis of polyvinyl alcohol are well-known and have found large-scale application in the form of staple fibres (cut fibres) in the production of, inter alia, paper coating, non-wovens and heavy yarns (canvas, etc.) and in the form of continuous fibres for separation purposes in textile production lines and for reinforcement of rubbers, plastics and other products, and furthermore as precursor of carbon fibres. The fibres used in these applications generally have a tensile strength of 0.3-1.2 GPa and a modulus of 3-30 GPa.
The method of preparation most commonly used for this type of fibre is the so-called wet spinning process. In this process, the polymer is dissolved in a suitable medium to obtain a concentration that is suitable for spinning, upon which the solution is forced through a spinneret ad the filaments obtained are passed through a coagulation bath, in which the solvent is removed and the polymer precipitates.
Besides the above-mentioned applications for synthetic polyvinyl alcohol fibres, there is increasing demand and a growing market for so-called technical-grade polyvinyl alcohol fibres, for instance for composite applications.
For these applications the fibres are to have a higher modulus and tensile strength.
Numerous methods have already been proposed, particularly in patent literature, for the preparation of polyvinyl alcohol fibres patent literature, for the preparation of polyvinyl alcohol fibres having improved mechanical properties.
From Applied Polymer Symposia No. 6 (1967), pp. 109-149, for instance, it is known to prepare fibres having a tensile strength of about 1.2 GPa by coagulation spinning (phase separation) of dilute solutions of polyvinyl alcohol in organic solvents, followed by stretching. From FR-A-1.280.192 it is known to prepare fibres having a maximum tensile strength of 1.7 GPa by coagulation followed by stretching, starting from solutions of polyvinyl alcohol in water in the presence of boric acid.
In the above-mentioned publications nothing is said about the moduli of the fibres obtained. Fro DE-B-2.132.055 it is known to convert a solution of polyvinyl alcohol in water into fibres having a maximum tensile strength of about 1.8 GPa and a maximum modulus of about 40 GPa through coagulation spinning and a special very laboricus multistage stetching operation with interim water treatment.
All the methods referred to start from a polyvinyl alcohol having a relatively low weight-average molecular weight, in general between 6.times.10.sup.4 and 12.times.10.sup.4, the fibres obtained having a strength and modulus that, though being higher than those of commercially available synthetic polyvinyl alcohol fibres, are not yet high enough for most technical applications.
It further is known to prepare fibres of very high tensile strength and modulus starting from solutions of polymers having a high molecular weight, see GB-B-2.042.414 and 2.051.667. According to the process described in GB-B-2.042.414, to this end a dilute solution of, for instance, polyethylene is spun, the filament obtained is cooled to a gel filament, and the solvent-containing gel filament is stretched at elevated temperature. According to the process described in GB-B-2.051.667 to this end a solution of high molecular weight polyethylene is spun, the solvent is optionally removed largely or partly, and the gel filament is subjected at a specific temperature to stretching at a high stretch ratio that is related to the molecular weight. In these known processes it has been found that the moduli, but especially the tensile strengths, that can be attained increase with the molecular weight of the polyethylene.
These known processes can, therefore, be used to prepare polyethylene based fibres having a tensile strength well in excess of 1.2 GPa and moduli of more than 20 GPa.
According to the above-mentioned GB-B-2.042.414 the process proposed in it can be used, inter alia, to prepare fibres having a high strength and modulus from polyvinyl alcohol. This is confirmed by, for instance, EP-A-105.169, which discloses a process for the preparation of fibres having a high tensile strength (1.3-2.2 GPa) and a high modulus (40-70 GPa) by spinning of a dilute solution of high molecular weight polyvinyl alcohol, with a weight-average molecular weight well in excess of 5.times.10.sup.5, in particular well in excess of 1.times.10.sup.6, cooling of the spun filament to obtain a gel filament, and stretching of this filament after extraction of the solvent.
According to EP-A-146.084 it is possible to prepare fibres of high strength and modulus starting from a polyvinyl alcohol having a relatively low molecular weight, by applying a special dry/wet coagulation spinning process, followed by cooling, solvent removal and drawing. A disadvantage of the process of EP-A-146.084 ad also of EP-A-105.169 is that the removal of solvent from the fibers by e.g. extraction is extremely difficult. In EP-A-105.169 an extraction time of 92 hours is mentioned. So both processes are not suitable for commercial operation.